With remarkable advances in developing electronic materials, many developments have been made in optically functional materials in the field of optoelectronics. For example, neodymium-containing glass has been introduced into actual use in laser beam electronic devices, whereas the glass has found only limited use because of the difficulties encountered in producing and working the glass and a high production cost. The dye laser, which can be used in the form of a solution, has the excellent features of being inexpensive, easy to work and cool and continuously usable for a prolonged period of time. However, the dye laser is low in strength and therefore limited in application.
On the other hand, organic optical materials such as polyphenylene have the drawback of being short in luminescence life and diminished in the effectiveness of luminescence although superior to inorganic optical materials in workability.
It is already known that a complex [Nd.sup.III (HFA).sub.3 ] comprising Nd.sup.3+ and three molecules of hexafluoroacetylacetone (HFA) coordinated therewith and having heavy hydrogen substituted for the hydrogen of active methylene luminesces in the state of a solution [S. Yanagida et al., J. Phys. Chem., 100, 10201(1996)]. This complex is short in luminescence life and low in quantum yield and decomposes in the presence of a very small amount of water to lose its optical conversion function. It has therefore been desired to develop complexes having a high luminescence intensity and good stability.
An object of the present invention is to provide rare-earth complexes having excellent luminescence characteristics, compounds useful as ligands for these complexes, optically functional materials containing the rare-earth complex and a process for preparing such rare-earth complexes.